What_is_Kohlrausch_Law.pdf

Explain Kohlrausch's Law?
Derivation of Kohlrausch's Law.
Some Important applications of Kohlrausch's Law.
and other uses of Kohlrausch's Law

1. What is Kohlrausch Law?
Kohlrausch’s law states that the equivalent conductivity of an electrolyte at
infinite dilution is equal to the sum of the conductances of the anions and
cations.
The molar conductivity of a solution at a given concentration is the
conductance of the volume of solution containing one mole of electrolyte kept
between two electrodes with the unit area of cross-section and distance of
unit length. The molar conductivity of a solution increases with the decrease in
concentration. This increase in molar conductivity is because of the increase in
the total volume containing one mole of the electrolyte. When the
concentration of the electrolyte approaches zero, the molar conductivity is
known as limiting molar conductivity, Ëm°.
Kohlrausch observed certain regularities while comparing the values of limiting
molar conductivities of some strong electrolytes. On the basis of his
observations, Kohlrausch proposed “limiting molar conductivity of an
electrolyte can be represented as the sum of the individual contributions of
the anions and cations of the electrolyte”. This law is popularly known as
Kohlrausch law of independent migration of ions. For example, limiting molar
conductivity, Ëm° of sodium chloride can be determined with the knowledge of
limiting molar conductivities of sodium ion and chloride ion.
Some important applications of Kohlrausch law of independent
migration of ions are:
1. Kohlrausch law helps us in the determination of limiting molar
conductivities for any electrolyte. Weak electrolytes have lower molar
conductivities and lower degree of dissociation at higher concentrations.
The graph plotted between molar conductivity and c1/2 (where c is the
concentration) is not a straight line for weak electrolytes. The molar
conductivity of weak electrolyte increases steeply at lower
concentrations. Therefore, limiting molar conductivity, Ëm° cannot be
obtained by extrapolation of molar conductivity to zero concentration.
Hence, we use the Kohlrausch law of independent migration of ions for

2. the determination of limiting molar conductivity, Ëm° for weak
electrolytes.
2. Kohlrausch law also helps us in determining the value of dissociation
constant from the value of molar conductivity and limiting molar
conductivity for a weak electrolyte at a given concentration.
α = ΛËm∘
Where, α = dissociation constant
Λ = molar conductivity
Ëm° = limiting molar conductivity

Other Uses of Kohlrausch’s law
• Calculation of Degree of dissociation
• Calculation of solubility of sparingly soluble salt
• Calculation of Dissociation Constant for week electrolytes
• Calculation of Molar Conductivity for week electrolytes at infinite
dilution